Given a positive externality, the marginal social benefits curve lies to the of the demand curve, with the market output the socially optimal output

Given a positive externality, the marginal social benefits curve lies to the right of the demand curve, with the market output below  the socially optimal output

The marginal benefits can be represented  by the demand curve. Assume a condition where the consumers are the only persons who gets benefits from the commodity. In this scenario, the demand curve will be the marginal benefit curve. When the society gets benefits by consuming an additional unit of a commodity production, then it is the marginal society benefits.

The marginal benefit curve and the demand curve will be same as long as you are willing to pay a next unit of a good. When you are willing to pay for an additional unit of good, it give rise to the marginal benefit curve. In the positive externality, the marginal benefits curve will be lying to the right of the demand curve, having the market output below  the socially optimal output.

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